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SMOG Index Calculator

Estimate reading difficulty by counting polysyllabic words

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The Gold Standard for Health Literacy Readability

SMOG stands for Simple Measure of Gobbledygook — a name that captures its creator's frustration with needlessly complex writing. The formula counts polysyllabic words (three or more syllables) relative to the number of sentences, then maps the result to a US grade level. It is deliberately simple: one word class to count, one formula to apply, one number to interpret.

How the Formula Works

SMOG normalizes the polysyllable count to a 30-sentence sample, takes the square root, and adds 3. The square root compresses the scale so that doubling the number of hard words does not double the grade level — a realistic reflection of how reading difficulty scales. The constant 3 anchors the bottom of the scale at about third-grade level, which is where polysyllabic words start appearing in school texts.

Why Healthcare Trusts SMOG

Most readability formulas predict the grade level at which 50 to 75 percent of readers will understand the text. SMOG was calibrated for 100 percent comprehension — every reader at the predicted grade level should understand the material. This makes it the right choice for consent forms, prescription labels, discharge instructions, and public health campaigns where partial understanding is not acceptable.

Lowering Your SMOG Score

Since SMOG counts only polysyllabic words, the most effective edit is replacing three-plus-syllable words with one- or two-syllable alternatives."Medication" becomes"medicine" or"drug.""Approximately" becomes"about.""Communicate" becomes"tell" or"share." The polysyllabic word list this tool provides is your editing checklist — go through it and ask whether each word has a simpler replacement your audience would prefer.

Combining SMOG with Other Formulas

SMOG tends to produce slightly higher grade levels than Flesch-Kincaid because of its 100-percent-comprehension target. If you run both and get different numbers, SMOG is the more conservative estimate. For general content, use the average of two to three formulas. For health and safety content, use SMOG alone and target the lower score it recommends.

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$ faq

What is the SMOG Index? ▸
The SMOG (Simple Measure of Gobbledygook) Index is a readability formula developed by G. Harry McLaughlin in 1969. It estimates the years of education a person needs to fully understand a piece of writing. SMOG is considered one of the most accurate readability formulas for health and medical content, which is why it is the preferred measure for healthcare communication in many countries. The output is a US grade level — a SMOG score of 10 means the text requires a tenth-grade reading level.
What is the SMOG formula? ▸
The standard formula is: SMOG = 3 + √(polysyllable count × 30 ÷ sentence count), where polysyllabic words are words with three or more syllables. The original method requires exactly 30 sentences — 10 from the beginning, 10 from the middle, and 10 from the end. For texts with fewer or more than 30 sentences, the formula normalizes by multiplying the polysyllable count by 30 divided by the actual sentence count. The square root and the constant 3 were derived from correlation with reading comprehension scores.
What SMOG score should I aim for? ▸
Patient-facing health materials should target SMOG 6 to 8, as recommended by the American Medical Association and the National Institutes of Health. General consumer content performs well at SMOG 8 to 10. Business and professional documents typically score 10 to 13. Academic writing often scores 13 and above. If your audience includes low-literacy readers, aim for SMOG 5 to 6. The US average adult reading level is around eighth grade, so keeping public-facing material at or below SMOG 8 ensures the widest reach.
Why is SMOG preferred for health and medical content? ▸
SMOG was validated specifically against reading comprehension tests with a focus on 100 percent comprehension rather than partial understanding. Most other formulas target 50 to 75 percent comprehension. This stricter standard makes SMOG particularly important for medical consent forms, medication instructions, and public health materials where misunderstanding can have serious consequences. The US Department of Health and Human Services, the CDC, and many hospital systems require SMOG assessments for patient materials.
What is a polysyllabic word? ▸
A polysyllabic word in the SMOG formula is any word with three or more syllables. Unlike the Gunning Fog Index, SMOG does not exclude proper nouns, compound words, or words inflated by suffixes. Every three-plus-syllable word counts equally. This simpler counting rule makes SMOG easier to apply consistently and less prone to subjective judgment about which words to include or exclude.
How many sentences do I need for an accurate SMOG score? ▸
The original SMOG method requires exactly 30 sentences for highest accuracy. With fewer sentences, the score is still useful but may be less stable — a single polysyllabic word has more influence on a short sample. Texts under 10 sentences should be treated as rough estimates. This calculator works with any length and normalizes the polysyllable count to a 30-sentence equivalent, which is the standard approach for varying text lengths.
Is any data sent to a server? ▸
No. All calculations run locally in JavaScript inside your browser. Your text is never transmitted or stored.